1. Field of the Invention
The present invention relates to a light emitting device, and in particular to forming a group-III nitride semiconductor layer thereon.
2. Description of the Related Art
A group-III nitride semiconductor light-emitting diode is fabricated by providing an electrode on a stacked layer structure having a pn-junction type light-emitting part comprising, for example, aluminum gallium indium nitride (AlxGayIn1-x-yN, where 0xe2x89xa6X, Yxe2x89xa61 and 0xe2x89xa6X+Yxe2x89xa61). In the stacked layer structure, a buffer layer is generally provided for relaxing lattice mismatch between the substrate material and the group-III nitride semiconductor layer constituting the stacked layer structure, thereby growing a high-quality group-III nitride semiconductor layer. Conventionally, for example, in the stacked layer structure for use in a light-emitting device using a sapphire (xcex1-Al2O3 single crystal) substrate, the buffer layer is exclusively composed of aluminum gallium nitride (compositional formula; Al60 Gaxcex2N, where 0xe2x89xa6xcex1, xcex2xe2x89xa61).
When a stacked layer structure uses an insulating material such as sapphire for the substrate, an electrode, namely, an ohmic electrode for supplying a device driving current to LED, comprising such a stacked layer structure is disposed on p-type and n-type conductive layers constituting the stacked layer structure.
In the unexamined published U.S. Pat. No. 2001/0,054,717, Takashi discloses a high emission intensity group-III nitride semiconductor light-emitting device obtained by eliminating crystal lattice mismatch with substrate crystal and using a gallium nitride phosphide-based light emitting structure having excellent crystallinity. A gallium nitride phosphide-based multilayer light-emitting structure is formed on a substrate via a boron phosphide (BP)-based buffer layer. The boron phosphide-based buffer layer is preferably grown at a low temperature and rendered amorphous to eliminate lattice mismatch with the substrate crystal. After the amorphous buffer layer is formed, it is gradually converted into a crystalline layer to fabricate a light-emitting device while keeping the lattice match with the gallium nitride phosphide-based light-emitting part.
Refinement of the group-III nitride semiconductor layer constituting the stacked layer structure is influenced by the quality of the buffer layer, since the lattice constant of the buffer layer and the lattice constant of group-III nitride based light-emitting layer can be approximated as close as possible by controlling the composition of the buffer layer. As the lattice mismatch is smaller, a good epitaxial crystal layer with reduced crystal defects can be more readily obtained, thereby contributing to the high-luminous intensity emission of the light-emitting device. If the buffer layer or the group-III nitride semiconductor layer contains too many defects, such as dislocations and particles, lighting efficiency and lifetime of the light-emitting device are reduced. In order to remove particles, nitrogen is usually introduced to clean the surface of the buffer layer. However, the surface of the buffer layer can be corrosiond by nitrogen. Although the particles are removed, still other defects, such as pin holes, occur on the surface of the buffer layer.
Accordingly, an object of the present invention is to provide a method of forming a group-III nitride semiconductor layer on a light-emitting device that comprises a surface treatment of the buffer layer to protect the buffer layer from corrosion and remove particles therefrom.
It is a further object of the present invention to provide a method of forming a pure group-III nitride semiconductor layer without dislocations on a light-emitting device to enhance lighting efficiency.
It is still another object of the present invention to provide a method of forming a pure group-III nitride semiconductor layer without dislocations on a light-emitting device to prolong lifetime.
The key feature of the present invention is hydrogen treatment on the buffer to prevent corrosion during subsequent process and remove particles therefrom, allowing enhanced structure for the epitaxy layer following its formation on the buffer layer.
To achieve these and other advantages, the invention provides a method of forming a group-III nitride semiconductor layer on a light-emitting device. First, a substrate is provided, and a buffer layer is formed thereon. A hydrogen treatment is performed on the buffer layer. Finally, a group-III nitride semiconductor layer is formed on the buffer layer.
Alternatively, according to the present invention, the substrate is removed before the hydrogen treatment.
The group-III nitride semiconductor, comprising at least a first cladding layer, an active layer, and a second cladding layer is subsequently formed on the buffer layer.